Electrolyte compositions for batteries

ABSTRACT

An energy storage device comprising a first electrode and a second electrode, a separator between the first electrode and the second electrode, and an electrolyte in contact with the first electrode, the second electrode, and the separator, wherein the electrolyte comprises at least one of a fluorine-containing cyclic carbonate, a fluorine-containing linear carbonate, and a fluoroether. The electrolyte may be substantially free of non-fluorine containing cyclic carbonates.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.14/800,380, filed Jul. 15, 2015, which is a continuation-in-part of U.S.application Ser. No. 13/799,405, filed Mar. 13, 2013, which is acontinuation-in-part of U.S. application Ser. No. 13/601,976, filed Aug.31, 2012, which claims the benefit of U.S. Provisional Application No.61/530,881, filed Sep. 2, 2011. U.S. application Ser. No. 13/601,976also is a continuation-in-part of U.S. application Ser. No. 13/008,800,filed Jan. 18, 2011, which claims the benefit of U.S. ProvisionalApplication Nos. 61/295,993, filed Jan. 18, 2010, and 61/315,845, filedMar. 19, 2010. The entirety of each of the above referenced applicationsis hereby incorporated by reference.

BACKGROUND Field

The present application relates generally to electrolytes and siliconparticles. In particular, the present application relates toelectrolytes and composite materials including silicon particles for usein battery electrodes.

Description of the Related Art

A lithium ion battery typically includes a separator and/or electrolytebetween an anode and a cathode. In one class of batteries, theseparator, cathode and anode materials are individually formed intosheets or films. Sheets of the cathode, separator and anode aresubsequently stacked or rolled with the separator separating the cathodeand anode (e.g., electrodes) to form the battery. Typical electrodesinclude electro-chemically active material layers on electricallyconductive metals (e.g., aluminum and copper). Films can be rolled orcut into pieces which are then layered into stacks. The stacks are ofalternating electro-chemically active materials with the separatorbetween them.

SUMMARY

In some embodiments, an energy storage device comprises a firstelectrode and a second electrode, a separator between the firstelectrode and the second electrode, and an electrolyte in contact withthe first electrode, the second electrode, and the separator, whereinthe electrolyte comprises at least one of a fluorine-containing cycliccarbonate, a fluorine-containing linear carbonate, and a fluoroether. Insome embodiments, the electrolyte may comprise fluoroethylene carbonate.At least one of the first electrode and the second electrode comprises aself-supporting composite material film, wherein the composite materialfilm comprising greater than 0% and less than about 90% by weight ofsilicon particles, and greater than 0% and less than about 90% by weightof one or more types of carbon phases, wherein at least one of the oneor more types of carbon phases is a substantially continuous phase thatholds the composite material film together such that the siliconparticles are distributed throughout the composite material film,

In some embodiments, an energy storage device comprises a firstelectrode and a second electrode, wherein at least one of the firstelectrode and the second electrode comprises a composite material film,the composite material film comprising greater than 0% and less thanabout 90% by weight of silicon particles, wherein the silicon particlescomprise an average particle size between about 10 nanometers and about40 microns, and greater than 0% and less than about 90% by weight of oneor more types of carbon phases, wherein at least one of the one or moretypes of carbon phases is a substantially continuous phase; a separatorbetween the first electrode and the second electrode; and an electrolytein contact with the first electrode, the second electrode, and theseparator, wherein the electrolyte comprises at least one of afluorine-containing cyclic carbonate, fluorine-containing linearcarbonate, and a fluoroether. In some embodiments, the electrolyte maycomprise fluoroethylene carbonate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an embodiment of a method of forming a compositematerial that includes forming a mixture that includes a precursor,casting the mixture, drying the mixture, curing the mixture, andpyrolyzing the precursor.

FIGS. 2A and 2B are SEM micrographs of one embodiment of micron-sizedsilicon particles milled-down from larger silicon particles.

FIGS. 2C and 2D are SEM micrographs of one embodiment of micron-sizedsilicon particles with nanometer-sized features on the surface.

FIG. 3 illustrates an example embodiment of a method of forming acomposite material.

FIG. 4 is a cross-sectional schematic diagram of an example of an energystorage device.

FIG. 5 is a plot of the discharge capacity at an average rate of C/2.6.

FIG. 6 is a plot of the discharge capacity at an average rate of C/3.

FIG. 7 is a plot of the discharge capacity at an average rate of C/3.3.

FIG. 8 is a plot of the discharge capacity at an average rate of C/5.

FIG. 9 is a plot of the discharge capacity at an average rate of C/9.

FIG. 10 is a plot of the discharge capacity.

FIG. 11 is a plot of the discharge capacity at an average rate of C/9.

FIGS. 12A and 12B are plots of the reversible and irreversible capacityas a function of the various weight percentage of PI derived carbon from2611c and graphite particles for a fixed percentage of 20 wt. % Si.

FIG. 13 is a plot of the first cycle discharge capacity as a function ofweight percentage of carbon.

FIG. 14 is a plot of the reversible (discharge) and irreversiblecapacity as a function of pyrolysis temperature.

FIG. 15 is a photograph of a 4.3 cm×4.3 cm composite anode film withouta metal foil support layer.

FIG. 16 is a scanning electron microscope (SEM) micrograph of acomposite anode film before being cycled (the out-of-focus portion is abottom portion of the anode and the portion that is in focus is acleaved edge of the composite film).

FIG. 17 is another SEM micrograph of a composite anode film before beingcycled.

FIG. 18 is a SEM micrograph of a composite anode film after being cycled10 cycles.

FIG. 19 is another SEM micrograph of a composite anode film after beingcycled 10 cycles.

FIG. 20 is a SEM micrograph of a composite anode film after being cycled300 cycles.

FIG. 21 includes SEM micrographs of cross-sections of composite anodefilms.

FIG. 22 is an x-ray powder diffraction (XRD) graph of the sample siliconparticles.

FIG. 23 is a SEM micrographs of one embodiment of silicon particles.

FIG. 24 is another SEM micrographs of one embodiment of siliconparticles.

FIG. 25 is a SEM micrographs of one embodiment of silicon particles.

FIG. 26 is a SEM micrographs of one embodiment of silicon particles.

FIG. 27 is a chemical analysis of the sample silicon particles.

FIGS. 28A and 28B are example particle size histograms of twomicron-sized silicon particles with nanometer-sized features.

FIG. 29 is a plot of discharge capacity during cell cycling comparingtwo types of example silicon particles.

FIG. 30 is a graph of examples of capacity retention performances,according to some embodiments.

FIG. 31 is a graph of additional examples of capacity retentionperformances, according to some embodiments.

FIGS. 32A-32C is a graph showing gas generation performances.

FIGS. 33A-33C is a graph showing some examples of gas generationperformance, according to some embodiments.

DETAILED DESCRIPTION

Typical carbon anode electrodes include a current collector such as acopper sheet. Carbon is deposited onto the collector along with aninactive binder material. Carbon is often used because it has excellentelectrochemical properties and is also electrically conductive. If thecurrent collector layer (e.g., copper layer) was removed, the carbonwould likely be unable to mechanically support itself. Therefore,conventional electrodes require a support structure such as thecollector to be able to function as an electrode. The electrode (e.g.,anode or cathode) compositions described in this application can produceelectrodes that are self-supported. The need for a metal foil currentcollector is eliminated or minimized because conductive carbonizedpolymer is used for current collection in the anode structure as well asfor mechanical support. In typical applications for the mobile industry,a metal current collector is typically added to ensure sufficient rateperformance. The carbonized polymer can form a substantially continuousconductive carbon phase in the entire electrode as opposed toparticulate carbon suspended in a non-conductive binder in one class ofconventional lithium-ion battery electrodes. Advantages of a carboncomposite blend that utilizes a carbonized polymer can include, forexample, 1) higher capacity, 2) enhanced overcharge/dischargeprotection, 3) lower irreversible capacity due to the elimination (orminimization) of metal foil current collectors, and 4) potential costsavings due to simpler manufacturing.

Anode electrodes currently used in the rechargeable lithium-ion cellstypically have a specific capacity of approximately 200 milliamp hoursper gram (including the metal foil current collector, conductiveadditives, and binder material). Graphite, the active material used inmost lithium ion battery anodes, has a theoretical energy density of 372milliamp hours per gram (mAh/g). In comparison, silicon has a hightheoretical capacity of 4200 mAh/g. In order to increase volumetric andgravimetric energy density of lithium-ion batteries, silicon may be usedas the active material for the cathode or anode. Several types ofsilicon materials, e.g., silicon nanopowders, silicon nanofibers, poroussilicon, and ball-milled silicon, have also been reported as viablecandidates as active materials for the negative or positive electrode.Small particle sizes (for example, sizes in the nanometer range)generally can increase cycle life performance. They also can displayvery high irreversible capacity. However, small particle sizes also canresult in very low volumetric energy density (for example, for theoverall cell stack) due to the difficulty of packing the activematerial. Larger particle sizes, (for example, sizes in the micrometeror micron range) generally can result in higher density anode material.However, the expansion of the silicon active material can result in poorcycle life due to particle cracking. For example, silicon can swell inexcess of 300% upon lithium insertion. Because of this expansion, anodesincluding silicon should be allowed to expand while maintainingelectrical contact between the silicon particles.

As described herein and in U.S. patent application Ser. Nos. 13/008,800and 13/601,976, entitled “Composite Materials for ElectrochemicalStorage” and “Silicon Particles for Battery Electrodes,” respectively,certain embodiments utilize a method of creating monolithic,self-supported anodes using a carbonized polymer. Because the polymer isconverted into a electrically conductive and electrochemically activematrix, the resulting electrode is conductive enough that a metal foilor mesh current collector can be omitted or minimized. The convertedpolymer also acts as an expansion buffer for silicon particles duringcycling so that a high cycle life can be achieved. In certainembodiments, the resulting electrode is an electrode that is comprisedsubstantially of active material. In further embodiments, the resultingelectrode is substantially active material. The electrodes can have ahigh energy density of between about 500 mAh/g to about 1200 mAh/g thatcan be due to, for example, 1) the use of silicon, 2) elimination orsubstantial reduction of metal current collectors, and 3) beingcomprised entirely or substantially entirely of active material.

The composite materials described herein can be used as an anode in mostconventional lithium ion batteries; they may also be used as the cathodein some electrochemical couples with additional additives. The compositematerials can also be used in either secondary batteries (e.g.,rechargeable) or primary batteries (e.g., non-rechargeable). In someembodiments, the composite materials can be used in batteriesimplemented as a pouch cell, as described in further details herein. Incertain embodiments, the composite materials are self-supportedstructures. In further embodiments, the composite materials areself-supported monolithic structures. For example, a collector may beincluded in the electrode comprised of the composite material. Incertain embodiments, the composite material can be used to form carbonstructures discussed in U.S. patent application Ser. No. 12/838,368entitled “Carbon Electrode Structures for Batteries,” the entirety ofwhich is hereby incorporated by reference. Furthermore, the compositematerials described herein can be, for example, silicon compositematerials, carbon composite materials, and/or silicon-carbon compositematerials. Certain embodiments described herein can further includecomposite materials including micron-sized silicon particles. Forexample, in some embodiments, the micron-sized silicon particles havenanometer-sized features on the surface. Silicon particles with such ageometry may have the benefits of both micron-sized silicon particles(e.g., high energy density) and nanometer-sized silicon particles (e.g.,good cycling behavior). As used herein, the term “silicon particles” ingeneral include micron-sized silicon particles with or withoutnanometer-sized features.

FIG. 1 illustrates one embodiment of a method of forming a compositematerial 100. For example, the method of forming a composite materialcan include forming a mixture including a precursor, block 101. Themethod can further include pyrolyzing the precursor to convert theprecursor to a carbon phase. The precursor mixture may include carbonadditives such as graphite active material, chopped or milled carbonfiber, carbon nanofibers, carbon nanotubes, and/or other carbons. Afterthe precursor is pyrolyzed, the resulting carbon material can be aself-supporting monolithic structure. In certain embodiments, one ormore materials are added to the mixture to form a composite material.For example, silicon particles can be added to the mixture. Thecarbonized precursor results in an electrochemically active structurethat holds the composite material together. For example, the carbonizedprecursor can be a substantially continuous phase. The siliconparticles, including micron-sized silicon particles with or withoutnanometer-sized features, may be distributed throughout the compositematerial. Advantageously, the carbonized precursor can be a structuralmaterial as well as an electro-chemically active and electricallyconductive material. In certain embodiments, material particles added tothe mixture are homogenously or substantially homogeneously distributedthroughout the composite material to form a homogeneous or substantiallyhomogeneous composite.

The mixture can include a variety of different components. The mixturecan include one or more precursors. In certain embodiments, theprecursor is a hydrocarbon compound. For example, the precursor caninclude polyamic acid, polyimide, etc. Other precursors can includephenolic resins, epoxy resins, and/or other polymers. The mixture canfurther include a solvent. For example, the solvent can beN-methyl-pyrrolidone (NMP). Other possible solvents include acetone,diethyl ether, gamma butyrolactone, isopropanol, dimethyl carbonate,ethyl carbonate, dimethoxyethane, ethanol, methanol, etc. Examples ofprecursor and solvent solutions include PI-2611 (HD Microsystems),PI-5878G (HD Microsystems) and VTEC PI-1388 (RBI, Inc.). PI-2611 iscomprised of >60% n-methyl-2-pyrrolidone and 10-30%s-biphenyldianhydride/p-phenylenediamine. PI-5878G is comprised of >60%n-methylpyrrolidone, 10-30% polyamic acid of pyromelliticdianhydride/oxydianiline, 10-30% aromatic hydrocarbon (petroleumdistillate) including 5-10% 1,2,4-trimethylbenzene. In certainembodiments, the amount of precursor in the solvent is about 10 wt. % toabout 30 wt. %. Additional materials can also be included in themixture. For example, as previously discussed, silicon particles orcarbon particles including graphite active material, chopped or milledcarbon fiber, carbon nanofibers, carbon nanotubes, and other conductivecarbons can be added to the mixture. In addition, the mixture can bemixed to homogenize the mixture.

In certain embodiments, the mixture is cast on a substrate, block 102 inFIG. 1. In some embodiments, casting includes using a gap extrusion or ablade casting technique. The blade casting technique can includeapplying a coating to the substrate by using a flat surface (e.g.,blade) which is controlled to be a certain distance above the substrate.A liquid or slurry can be applied to the substrate, and the blade can bepassed over the liquid to spread the liquid over the substrate. Thethickness of the coating can be controlled by the gap between the bladeand the substrate since the liquid passes through the gap. As the liquidpasses through the gap, excess liquid can also be scraped off. Forexample, the mixture can be cast on a substrate comprising a polymersheet, a polymer roll, and/or foils or rolls made of glass or metal. Themixture can then be dried to remove the solvent, block 103. For example,a polyamic acid and NMP solution can be dried at about 110° C. for about2 hours to remove the NMP solution. The dried mixture can then beremoved from the substrate. For example, an aluminum substrate can beetched away with HCl. Alternatively, the dried mixture can be removedfrom the substrate by peeling or otherwise mechanically removing thedried mixture from the substrate. In some embodiments, the substratecomprises polyethylene terephthalate (PET), including for exampleMylar®. In certain embodiments, the dried mixture is a film or sheet. Insome embodiments, the dried mixture is cured, block 104. A hot press canbe used to cure and to keep the dried mixture flat. For example, thedried mixture from a polyamic acid and NMP solution can be hot pressedat about 200° C. for about 8 to 16 hours. Alternatively, the entireprocess including casting and drying can be done as a roll-to-rollprocess using standard film-handling equipment. The dried mixture can berinsed to remove any solvents or etchants that may remain. For example,de-ionized (DI) water can be used to rinse the dried mixture. In certainembodiments, tape casting techniques can be used for the casting. Inother embodiments, there is no substrate for casting and the anode filmdoes not need to be removed from any substrate. The dried mixture may becut or mechanically sectioned into smaller pieces.

The mixture further goes through pyrolysis to convert the polymerprecursor to carbon, block 105. In certain embodiments, the mixture ispyrolysed in a reducing atmosphere. For example, an inert atmosphere, avacuum and/or flowing argon, nitrogen, or helium gas can be used. Insome embodiments, the mixture is heated to about 900° C. to about 1350°C. For example, polyimide formed from polyamic acid can be carbonized atabout 1175° C. for about one hour. In certain embodiments, the heat uprate and/or cool down rate of the mixture is about 10° C./min. A holdermay be used to keep the mixture in a particular geometry. The holder canbe graphite, metal, etc. In certain embodiments, the mixture is heldflat. After the mixture is pyrolysed, tabs can be attached to thepyrolysed material to form electrical contacts. For example, nickel,copper or alloys thereof can be used for the tabs.

In certain embodiments, one or more of the methods described herein canbe carried out in a continuous process. In certain embodiments, casting,drying, curing and pyrolysis can be performed in a continuous process.For example, the mixture can be coated onto a glass or metal cylinder.The mixture can be dried while rotating on the cylinder to create afilm. The film can be transferred as a roll or peeled and fed intoanother machine for further processing. Extrusion and other filmmanufacturing techniques known in industry could also be utilized priorto the pyrolysis step.

Pyrolysis of the precursor results in a carbon material (e.g., at leastone carbon phase). In certain embodiments, the carbon material is a hardcarbon. In some embodiments, the precursor is any material that can bepyrolysed to form a hard carbon. When the mixture includes one or moreadditional materials or phases in addition to the carbonized precursor,a composite material can be created. In particular, the mixture caninclude silicon particles, creating a silicon-carbon (e.g., at least onefirst phase comprising silicon and at least one second phase comprisingcarbon) or silicon-carbon-carbon (e.g., at least one first phasecomprising silicon, at least one second phase comprising carbon, and atleast one third phase comprising carbon) composite material. Siliconparticles can increase the specific lithium insertion capacity of thecomposite material. When silicon absorbs lithium ions, it experiences alarge volume increase on the order of 300+ volume percent which cancause electrode structural integrity issues. In addition to volumetricexpansion related problems, silicon is not inherently electricallyconductive, but becomes conductive when it is alloyed with lithium(e.g., lithiation). When silicon de-lithiates, the surface of thesilicon loses electrical conductivity. Furthermore, when siliconde-lithiates, the volume decreases which results in the possibility ofthe silicon particle losing contact with the matrix. The dramatic changein volume also results in mechanical failure of the silicon particlestructure, in turn, causing it to pulverize. Pulverization and loss ofelectrical contact have made it a challenge to use silicon as an activematerial in lithium-ion batteries. A reduction in the initial size ofthe silicon particles can prevent further pulverization of the siliconpowder as well as minimizing the loss of surface electricalconductivity. Furthermore, adding material to the composite that canelastically deform with the change in volume of the silicon particlescan ensure that electrical contact to the surface of the silicon is notlost. For example, the composite material can include carbons such asgraphite which contributes to the ability of the composite to absorbexpansion and which is also capable of intercalating lithium ions addingto the storage capacity of the electrode (e.g., chemically active).Therefore, the composite material may include one or more types ofcarbon phases.

In some embodiments, a largest dimension of the silicon particles can beless than about 40 μm, less than about 1 μm, between about 10 nm andabout 40 μm, between about 10 nm and about 1 μm, less than about 500 nm,less than about 100 nm, and about 100 nm. All, substantially all, or atleast some of the silicon particles may comprise the largest dimensiondescribed above. For example, an average or median largest dimension ofthe silicon particles can be less than about 40 μm, less than about 1μm, between about 10 nm and about 40 μm, between about 10 nm and about 1μm, less than about 500 nm, less than about 100 nm, and about 100 nm.The amount of silicon in the composite material can be greater than zeropercent by weight of the mixture and composite material. In certainembodiments, the mixture comprises an amount of silicon, the amountbeing within a range of from about 0% to about 90% by weight, includingfrom about 30% to about 80% by weight of the mixture. The amount ofsilicon in the composite material can be within a range of from about 0%to about 35% by weight, including from about 0% to about 25% by weight,from about 10% to about 35% by weight, and about 20% by weight. Infurther certain embodiments, the amount of silicon in the mixture is atleast about 30% by weight. Additional embodiments of the amount ofsilicon in the composite material include more than about 50% by weight,between about 30% and about 80% by weight, between about 50% and about70% by weight, between about 60% and about 80% by weight, and betweenabout 70% and about 80% by weight. Furthermore, the silicon particlesmay or may not be pure silicon. For example, the silicon particles maybe substantially silicon or may be a silicon alloy. In one embodiment,the silicon alloy includes silicon as the primary constituent along withone or more other elements.

As described herein, micron-sized silicon particles can provide goodvolumetric and gravimetric energy density combined with good cycle life.In certain embodiments, to obtain the benefits of both micron-sizedsilicon particles (e.g., high energy density) and nanometer-sizedsilicon particles (e.g., good cycle behavior), silicon particles canhave an average particle size in the micron range and a surfaceincluding nanometer-sized features. In some embodiments, the siliconparticles have an average particle size (e.g., average diameter oraverage largest dimension) between about 0.1 μm and about 30 μm orbetween about 0.1 μm and all values up to about 30 μm. For example, thesilicon particles can have an average particle size between about 0.5 μmand about 25 μm, between about 0.5 μm and about 20 μm, between about 0.5μm and about 15 μm, between about 0.5 μm and about 10 μm, between about0.5 μm and about 5 μm, between about 0.5 μm and about 2 μm, betweenabout 1 μm and about 20 μm, between about 1 μm and about 15 μm, betweenabout 1 μm and about 10 μm, between about 5 μm and about 20 μm, etc.Thus, the average particle size can be any value between about 0.1 μmand about 30 μm, e.g., 0.1 μm, 0.5 μm, 1 μm, 5 μm, 10 μm, 15 μm, 20 μm,25 μm, and 30 μm.

The nanometer-sized features can include an average feature size (e.g.,an average diameter or an average largest dimension) between about 1 nmand about 1 μm, between about 1 nm and about 750 nm, between about 1 nmand about 500 nm, between about 1 nm and about 250 nm, between about 1nm and about 100 nm, between about 10 nm and about 500 nm, between about10 nm and about 250 nm, between about 10 nm and about 100 nm, betweenabout 10 nm and about 75 nm, or between about 10 nm and about 50 nm. Thefeatures can include silicon.

The amount of carbon obtained from the precursor can be about 50 weightpercent from polyamic acid. In certain embodiments, the amount of carbonfrom the precursor in the composite material is about 10% to about 25%by weight. The carbon from the precursor can be hard carbon. Hard carboncan be a carbon that does not convert into graphite even with heating inexcess of 2800 degrees Celsius. Precursors that melt or flow duringpyrolysis convert into soft carbons and/or graphite with sufficienttemperature and/or pressure. Hard carbon may be selected since softcarbon precursors may flow and soft carbons and graphite aremechanically weaker than hard carbons. Other possible hard carbonprecursors can include phenolic resins, epoxy resins, and other polymersthat have a very high melting point or are crosslinked. In someembodiments, the amount of hard carbon in the composite material has avalue within a range of from about 10% to about 25% by weight, about 20%by weight, or more than about 50% by weight. In certain embodiments, thehard carbon phase is substantially amorphous. In other embodiments, thehard carbon phase is substantially crystalline. In further embodiments,the hard carbon phase includes amorphous and crystalline carbon. Thehard carbon phase can be a matrix phase in the composite material. Thehard carbon can also be embedded in the pores of the additives includingsilicon. The hard carbon may react with some of the additives to createsome materials at interfaces. For example, there may be a siliconcarbide layer between silicon particles and the hard carbon.

In certain embodiments, graphite particles are added to the mixture.Advantageously, graphite can be an electrochemically active material inthe battery as well as an elastic deformable material that can respondto volume change of the silicon particles. Graphite is the preferredactive anode material for certain classes of lithium-ion batteriescurrently on the market because it has a low irreversible capacity.Additionally, graphite is softer than hard carbon and can better absorbthe volume expansion of silicon additives. In certain embodiments, alargest dimension of the graphite particles is between about 0.5 micronsand about 20 microns. All, substantially all, or at least some of thegraphite particles may comprise the largest dimension described herein.In further embodiments, an average or median largest dimension of thegraphite particles is between about 0.5 microns and about 20 microns. Incertain embodiments, the mixture includes greater than 0% and less thanabout 80% by weight of graphite particles, greater than about 5% andless than about 50% by weight of graphite particles, greater than about5% and less than about 40% by weight of graphite particles, greater thanabout 5% and less than about 30% by weight of graphite particles,greater than about 5% and less than about 20% by weight of graphiteparticles, and greater than about 5% and less than about 15% by weightof graphite particles. In further embodiments, the composite materialincludes about 40% to about 75% by weight graphite particles.Accordingly, in certain embodiments, the composite material can comprisesilicon particles, hard carbon, and graphite particles in anycombination of the ranges described herein. For example, the compositematerial can comprise about 60% to about 80% by weight of siliconparticles, about 10% to about 25% hard carbon, and about 5% to about 20%graphite particles. Other examples are possible.

In certain embodiments, conductive particles which may also beelectrochemically active are added to the mixture. Such particles canenable both a more electronically conductive composite as well as a moremechanically deformable composite capable of absorbing the largevolumetric change incurred during lithiation and de-lithiation. Incertain embodiments, a largest dimension of the conductive particles isbetween about 10 nanometers and about 7 millimeters. All, substantiallyall, or at least some of the conductive particles may comprise thelargest dimension described herein. In further embodiments, an averageor median largest dimension of the conductive particles is between about10 nm and about 7 millimeters. In certain embodiments, the mixtureincludes greater than zero and up to about 80% by weight conductiveparticles. In further embodiments, the composite material includes about45% to about 80% by weight conductive particles. The conductiveparticles can be conductive carbon including carbon blacks, carbonfibers, carbon nanofibers, carbon nanotubes, etc. Many carbons that areconsidered as conductive additives that are not electrochemically activebecome active once pyrolyzed in a polymer matrix. Alternatively, theconductive particles can be metals or alloys including copper, nickel,or stainless steel.

In certain embodiments, an electrode can include a composite materialdescribed herein. For example, a composite material can form aself-supported monolithic electrode. The pyrolysed carbon phase (e.g.,hard carbon phase) of the composite material can hold together andstructurally support the particles that were added to the mixture. Incertain embodiments, the self-supported monolithic electrode does notinclude a separate collector layer and/or other supportive structures.In some embodiments, the composite material and/or electrode does notinclude a polymer beyond trace amounts that remain after pyrolysis ofthe precursor. In further embodiments, the composite material and/orelectrode does not include a non-electrically conductive binder. Thecomposite material may also include porosity. For example, the porositycan be about 5% to about 40% by volume porosity.

The composite material may also be formed into a powder. For example,the composite material can be ground into a powder. The compositematerial powder can be used as an active material for an electrode. Forexample, the composite material powder can be deposited on a collectorin a manner similar to making a conventional electrode structure, asknown in the industry.

In certain embodiments, an electrode in a battery or electrochemicalcell can include a composite material, including composite material withthe silicon particles described herein. For example, the compositematerial can be used for the anode and/or cathode. In certainembodiments, the battery is a lithium ion battery. In furtherembodiments, the battery is a secondary battery, or in otherembodiments, the battery is a primary battery.

Furthermore, the full capacity of the composite material may not beutilized during use of the battery to improve life of the battery (e.g.,number charge and discharge cycles before the battery fails or theperformance of the battery decreases below a usability level). Forexample, a composite material with about 70% by weight siliconparticles, about 20% by weight carbon from a precursor, and about 10% byweight graphite may have a maximum gravimetric capacity of about 2000mAh/g, while the composite material may only be used up to angravimetric capacity of about 550 to about 850 mAh/g. Although, themaximum gravimetric capacity of the composite material may not beutilized, using the composite material at a lower capacity can stillachieve a higher capacity than certain lithium ion batteries. In certainembodiments, the composite material is used or only used at angravimetric capacity below about 70% of the composite material's maximumgravimetric capacity. For example, the composite material is not used atan gravimetric capacity above about 70% of the composite material'smaximum gravimetric capacity. In further embodiments, the compositematerial is used or only used at an gravimetric capacity below about 50%of the composite material's maximum gravimetric capacity or below about30% of the composite material's maximum gravimetric capacity.

Silicon Particles

Described herein are silicon particles for use in battery electrodes(e.g., anodes and cathodes). Anode electrodes currently used in therechargeable lithium-ion cells typically have a specific capacity ofapproximately 200 milliamp hours per gram (including the metal foilcurrent collector, conductive additives, and binder material). Graphite,the active material used in most lithium ion battery anodes, has atheoretical energy density of 372 milliamp hours per gram (mAh/g). Incomparison, silicon has a high theoretical capacity of 4200 mAh/g.Silicon, however, swells in excess of 300% upon lithium insertion.Because of this expansion, anodes including silicon should be able toexpand while allowing for the silicon to maintain electrical contactwith the silicon.

Some embodiments provide silicon particles that can be used as anelectro-chemically active material in an electrode. The electrode mayinclude binders and/or other electro-chemically active materials inaddition to the silicon particles. For example, the silicon particlesdescribed herein can be used as the silicon particles in the compositematerials described herein. In another example, an electrode can have anelectro-chemically active material layer on a current collector, and theelectro-chemically active material layer includes the silicon particles.The electro-chemically active material may also include one or moretypes of carbon.

Advantageously, the silicon particles described herein can improveperformance of electro-chemically active materials such as improvingcapacity and/or cycling performance. Furthermore, electro-chemicallyactive materials having such silicon particles may not significantlydegrade as a result of lithiation of the silicon particles.

In certain embodiments, the silicon particles have an average particlesize, for example an average diameter or an average largest dimension,between about 10 nm and about 40 μm. Further embodiments include averageparticle sizes of between about 1 μm and about 15 μm, between about 10nm and about 1 μm, and between about 100 nm and about 10 μm. Siliconparticles of various sizes can be separated by various methods such asby air classification, sieving or other screening methods. For example,a mesh size of 325 can be used separate particles that have a particlesize less than about 44 μm from particles that have a particle sizegreater than about 44 μm.

Furthermore, the silicon particles may have a distribution of particlesizes. For example, at least about 90% of the particles may haveparticle size, for example a diameter or a largest dimension, betweenabout 10 nm and about 40 μm, between about 1 μm and about 15 μm, betweenabout 10 nm and about 1 μm, and/or larger than 200 nm.

In some embodiments, the silicon particles may have an average surfacearea per unit mass of between about 1 to about 100 m²/g, about 1 toabout 80 m²/g, about 1 to about 60 m²/g, about 1 to about 50 m²/g, about1 to about 30 m²/g, about 1 to about 10 m²/g, about 1 to about 5 m²/g,about 2 to about 4 m²/g, or less than about 5 m²/g.

In certain embodiments, the silicon particles are at least partiallycrystalline, substantially crystalline, and/or fully crystalline.Furthermore, the silicon particles may be substantially pure silicon.

Compared with the silicon particles used in conventional electrodes, thesilicon particles described herein generally have a larger averageparticle size. In some embodiments, the average surface area of thesilicon particles described herein is generally smaller. Without beingbound to any particular theory, the lower surface area of the siliconparticles described herein may contribute to the enhanced performance ofelectrochemical cells. Typical lithium ion type rechargeable batteryanodes would contain nano-sized silicon particles. In an effort tofurther increase the capacity of the cell, smaller silicon particles(such as those in nano-size ranges) are being used for making theelectrode active materials. In some cases, the silicon particles aremilled to reduce the size of the particles. Sometimes the milling mayresult in roughened or scratched particle surface, which also increasesthe surface area. However, the increased surface area of siliconparticles may actually contribute to increased degradation ofelectrolytes, which lead to increased irreversible capacity loss. FIGS.2A and 2B are SEM micrographs of an example embodiment of siliconparticles milled-down from larger silicon particles. As shown in thefigures, certain embodiments may have a roughened surface.

As described herein, certain embodiments include silicon particles withsurface roughness in nanometer-sized ranges, e.g., micron-sized siliconparticles with nanometer-sized features on the surface. FIGS. 2C and 2Dare SEM micrographs of an example embodiment of such silicon particles.Various such silicon particles can have an average particle size (e.g.,an average diameter or an average largest dimension) in the micron range(e.g., as described herein, between about 0.1 μm and about 30 μm) and asurface including nanometer-sized features (e.g., as described herein,between about 1 nm and about 1 μm, between about 1 nm and about 750 nm,between about 1 nm and about 500 nm, between about 1 nm and about 250nm, between about 1 nm and about 100 nm, between about 10 nm and about500 nm, between about 10 nm and about 250 nm, between about 10 nm andabout 100 nm, between about 10 nm and about 75 nm, or between about 10nm and about 50 nm). The features can include silicon.

Compared to the example embodiment shown in FIGS. 2A and 2B, siliconparticles with a combined micron/nanometer-sized geometry (e.g., FIGS.2C and 2D) can have a higher surface area than milled-down particles.Thus, the silicon particles to be used can be determined by the desiredapplication and specifications.

Even though certain embodiments of silicon particles havenanometer-sized features on the surface, the total surface area of theparticles can be more similar to micron-sized particles than tonanometer-sized particles. For example, micron-sized silicon particles(e.g., silicon milled-down from large particles) typically have anaverage surface area per unit mass of over about 0.5 m²/g and less thanabout 2 m²/g (for example, using Brunauer Emmet Teller (BET) particlesurface area measurements), while nanometer-sized silicon particlestypically have an average surface area per unit mass of above about 100m²/g and less than about 500 m²/g. Certain embodiments described hereincan have an average surface area per unit mass between about 1 m²/g andabout 30 m²/g, between about 1 m²/g and about 25 m²/g, between about 1m²/g and about 20 m²/g, between about 1 m²/g and about 10 m²/g, betweenabout 2 m²/g and about 30 m²/g, between about 2 m²/g and about 25 m²/g,between about 2 m²/g and about 20 m²/g, between about 2 m²/g and about10 m²/g, between about 3 m²/g and about 30 m²/g, between about 3 m²/gand about 25 m²/g, between about 3 m²/g and about 20 m²/g, between about3 m²/g and about 10 m²/g (e.g., between about 3 m²/g and about 6 m²/g),between about 5 m²/g and about 30 m²/g, between about 5 m²/g and about25 m²/g, between about 5 m²/g and about 20 m²/g, between about 5 m²/gand about 15 m²/g, or between about 5 m²/g and about 10 m²/g.

Various examples of micron-sized silicon particles with nanometer-sizedfeatures can be used to form certain embodiments of composite materialsas described herein. For example, FIG. 3 illustrates an example method200 of forming certain embodiments of the composite material. The method200 includes providing a plurality of silicon particles (for example,silicon particles having an average particle size between about 0.1 μmand about 30 μm and a surface including nanometer-sized features), block210. The method 200 further includes forming a mixture that includes aprecursor and the plurality of silicon particles, block 220. The method200 further includes pyrolysing the precursor, block 230, to convert theprecursor into one or more types of carbon phases to form the compositematerial.

With respect to block 210 of method 200, silicon with thecharacteristics described herein can be synthesized as a product orbyproduct of a Fluidized Bed Reactor (FBR) process. For example, in theFBR process, useful material can be grown on seed silicon material.Typically, particles can be removed by gravity from the reactor. Somefine particulate silicon material can exit the reactor from the top ofthe reactor or can be deposited on the walls of the reactor. Thematerial that exits the top of the reactor or is deposited on the wallsof the reactor (e.g., byproduct material) can have nanoscale features ona microscale particle. In some such processes, a gas (e.g., a nitrogencarrier gas) can be passed through the silicon material. For example,the silicon material can be a plurality of granular silicon. The gas canbe passed through the silicon material at high enough velocities tosuspend the solid silicon material and make it behave as a fluid. Theprocess can be performed under an inert atmosphere, e.g., under nitrogenor argon. In some embodiments, silane gas can also be used, for example,to allow for metal silicon growth on the surface of the siliconparticles. The growth process from a gas phase can give the siliconparticles the unique surface characteristics, e.g., nanometer-sizedfeatures. Since silicon usually cleaves in a smooth shape, e.g., likeglass, certain embodiments of silicon particles formed using the FBRprocess can advantageously acquire small features, e.g., innanometer-sized ranges, that may not be as easily achievable in someembodiments of silicon particles formed by milling from larger siliconparticles.

In addition, since the FBR process can be under an inert atmosphere,very high purity particles (for example, higher than 99.9999% purity)can be achieved. In some embodiments, purity of between about 99.9999%and about 99.999999% can be achieved. In some embodiments, the FBRprocess can be similar to that used in the production of solar-gradepolysilicon while using 85% less energy than the traditional Siemensmethod, where polysilicon can be formed as trichlorosilane decomposesand deposits additional silicon material on high-purity silicon rods at1150° C. Because nanometer-sized silicon particles have been shown toincrease cycle life performance in electrochemical cells, micron-sizedsilicon particles have not been contemplated for use as electrochemicalactive materials in electrochemical cells.

With respect to blocks 220 and 230 of method 200, forming a mixture thatincludes a precursor and the plurality of silicon particles, block 220,and pyrolysing the precursor, block 230, to convert the precursor intoone or more types of carbon phases to form the composite material can besimilar to blocks 101 and 105 respectively, of method 100 describedherein. In some embodiments, pyrolysing (e.g., at about 900° C. to about1350° C.) occurs at temperatures below the melting point of silicon(e.g., at about 1414° C.) without affecting the nanometer-sized featuresof the silicon particles.

In accordance with certain embodiments described herein, certainmicron-sized silicon particles with nanometer surface feature canachieve high energy density, and can be used in composite materialsand/or electrodes for use in electro-chemical cells to improveperformance during cell cycling.

Electrolyte

An electrolyte for a lithium ion battery can include a solvent and alithium ion source, such as a lithium-containing salt. The compositionof the electrolyte may be selected to provide a lithium ion battery withimproved performance. In some embodiments, the electrolyte may containfluoroethylene carbonate. As described herein, a lithium ion battery mayinclude a first electrode, a second electrode, a separator between thefirst electrode and the second electrode, and an electrolyte in contactwith the first electrode, the second electrode, and the separator. Theelectrolyte serves to facilitate ionic transport between the firstelectrode and the second electrode. In some embodiment, the firstelectrode and the second electrode can refer to anode and cathode orcathode and anode, respectively.

In some embodiments, the electrolyte for a lithium ion battery mayinclude a solvent comprising a fluorine-containing component, such as afluorine-containing cyclic carbonate, a fluorine-containing linearcarbonate, and/or a fluoroether. In some embodiments, the electrolytecan include more than one solvent. For example, the electrolyte mayinclude two or more co-solvents. In some embodiments, at least one ofthe co-solvents in the electrolyte is a fluorine-containing compound. Insome embodiments, the fluorine-containing compound may be fluoroethylenecarbonate (FEC), 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropylether, or difluoroethylene carbonate (F2EC). In some embodiments, theco-solvent may be selected from the group consisting of fluoroethylenecarbonate (FEC), ethyl methyl carbonate (EMC), 1,1,2,2-tetrafluoroethyl2,2,3,3-tetrafluoropropyl ether, difluoroethylene carbonate (F2EC),ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate(DMC), propylene carbonate (PC), and gamma-Butyrolactone (GBL). In someembodiments, the electrolyte contains FEC. In some embodiments, theelectrolyte contains both EMC and FEC. In some embodiments, theelectrolyte may further contain 1,1,2,2-tetrafluoroethyl2,2,3,3-tetrafluoropropyl ether, EC, DEC, DMC, PC, GBL, and/or F2EC as aco-solvent. In some embodiments, the electrolyte is free orsubstantially free of non-fluorine-containing cyclic carbonates, such asEC, GBL, and PC.

As used herein, a co-solvent of an electrolyte has a concentration of atleast about 10% by volume (vol %). In some embodiments, a co-solvent ofthe electrolyte may be about 20 vol %, about 40 vol %, about 60 vol %,or about 80 vol %, or about 90 vol % of the electrolyte. In someembodiments, a co-solvent may have a concentration from about 10 vol %to about 90 vol %, from about 10 vol % to about 80 vol %, from about 10vol % to about 60 vol %, from about 20 vol % to about 60 vol %, fromabout 20 vol % to about 50 vol %, from about 30 vol % to about 60 vol %,or from about 30 vol % to about 50 vol %.

For example, in some embodiments, the electrolyte may contain afluorine-containing cyclic carbonate, such as FEC, at a concentration ofabout 10 vol % to about 60 vol %, including from about 20 vol % to about50 vol %, and from about 20 vol % to about 40 vol %. In someembodiments, the electrolyte may comprise a linear carbonate that doesnot contain flourine, such as EMC, at a concentration of about 40 vol %to about 90 vol %, including from about 50 vol % to about 80 vol %, andfrom about 60 vol % to about 80 vol %. In some embodiments, theelectrolyte may comprise 1,1,2,2-tetrafluoroethyl2,2,3,3-tetrafluoropropyl ether at a concentration of from about 10 vol% to about 30 vol %, including from about 10 vol % to about 20 vol %.

In some embodiments, the electrolyte is substantially free of cycliccarbonates other than fluorine-containing cyclic carbonates. Examples ofnon-fluorine-containing carbonates include ethylene carbonate (EC),propylene carbonate (PC), gamma-Butyrolactone (GBL), and vinylenecarbonate (VC). These non-fluorine-containing cyclic carbonatecompounds, along with some additives, can react with silicon on an anodeto form a solid electrolyte interface layer that can crack and/orcontinue to grow as the silicon containing anode expands and contractsduring cycling. Without being bound to the theory or mode of operation,it is believed that the presence of fluorine-containing cyclic carbonateand/or fluoroether and minimization of non-fluorine-containing cycliccarbonates in the electrolyte can result in a solid electrolyteinterface layer high in lithium fluoride (LiF) content. A solidelectrolyte interface layer comprising LiF may demonstrate improvedchemical stability and increased density, for example, compared to solidelectrolyte interface layers formed by other cyclic carbonates. As such,the change in thickness and surface reactivity of the interface layerare limited, which may in turn facilitate reduction in capacity fadeand/or generation of excessive gaseous byproducts during operation ofthe lithium ion battery. In some embodiments, electrolyte solventscomprising significant quantities of fluorinated compounds may be lessflammable.

In some embodiments, the electrolyte may further comprise one or moreadditives. As used herein, an additive of the electrolyte refers to acomponent that makes up less than 10% by weight (wt %) of theelectrolyte. In some embodiments, the amount of each additive in theelectrolyte may be from about 1 wt % to about 9 wt %, from about 1 wt %to about 8 wt %, from about 1 wt % to about 8 wt %, from about 1 wt % toabout 7 wt %, from about 1 wt % to about 6 wt %, from about 1 wt % toabout 5 wt %, from about 2 wt % to about 5 wt %, or any value inbetween. In some embodiments, the total amount of the additive(s) may befrom from about 1 wt % to about 9 wt %, from about 1 wt % to about 8 wt%, from about 1 wt % to about 7 wt %, from about 2 wt % to about 7 wt %,or any value in between. The additive may be selected from the groupconsisting of: 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether(D2), adiponitrile (AN), lithium difluoro(oxalato)borate (LiDFOB),trimethoxymethylsilane (MTMS), 1,3 propanesultone (PS), trimethylphosphate (TMP), and succinonitrile (SN). In some embodiments,1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether may be includedin the electrolyte as a co-solvent. In other embodiments,1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether may be includedin the electrolyte as an additive. For example, the electrolyte maycontain 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether as aco-solvent at a concentration of about 10 vol % or more. In otherembodiments, the electrolyte may contain 1,1,2,2-tetrafluoroethyl2,2,3,3-tetrafluoropropyl ether as an additive at less than 10 weight %.

In some embodiments, a lithium-containing salt for a lithium ion batterymay comprise lithium hexafluorophosphate (LiPF₆). In some embodiments, alithium-containing salt for a lithium ion battery may comprise one ormore of lithium tetrafluoroborate (LiBF₄), lithium hexafluoroarsenatemonohydrate (LiAsF₆), lithium perchlorate (LiClO₄), and lithium triflate(LiCF₃SO₃). In some embodiments, the electrolyte can have a saltconcentration of about 1 moles/L (M).

In some embodiments, a lithium ion battery comprising an electrolytecomposition according to one or more embodiments described herein, andan anode having a composite electrode film according to one or moreembodiments described herein, may demonstrate reduced gassing and/orswelling at about room temperature (e.g., about 20° C. to about 25° C.)or elevated temperatures (e.g., up to temperatures of about 85° C.),increased cycle life at about room temperature or elevated temperatures,and/or reduced cell growth/electrolyte consumption per cycle, forexample compared to lithium ion batteries comprising conventionallyavailable electrolyte compositions in combination with an anode having acomposite electrode film according to one or more embodiments describedherein. In some embodiments, a lithium ion battery comprising anelectrolyte composition according to one or more embodiments describedherein and an anode having a composite electrode film according to oneor more embodiments described herein may demonstrate reduced gassingand/or swelling across various temperatures at which the battery may besubject to testing, such as temperatures between about −20° C. and about130° C. (e.g., compared to lithium ion batteries comprisingconventionally available electrolyte compositions in combination with ananode having a composite electrode film according to one or moreembodiments described herein).

Gaseous byproducts may be undesirably generated during batteryoperation, for example, due to chemical reactions between theelectrolyte and one or more other components of the lithium ion battery,such as one or more components of a battery electrode. Excessive gasgeneration during operation of the lithium ion battery may adverselyaffect battery performance and/or result in mechanical and/or electricalfailure of the battery. For example, undesired chemical reactionsbetween an electrolyte and one or more components of an anode may resultin gas generation at levels which can mechanically (e.g., structuraldeformation) and/or electrochemically degrade the battery. In someembodiments, the composition of the anode and the composition of theelectrolyte can be selected to facilitate desired gas generationperformance.

Pouch Cell

As described herein, a battery can be implement as a pouch cell. FIG. 4shows a cross-sectional schematic diagram of an example of a lithium ionbattery 300 implemented as a pouch cell, according to some embodiments.The battery 300 comprises an anode 316 in contact with a negativecurrent collector 308, a cathode 304 in contact with a positive currentcollector 310, a separator 306 disposed between the anode 316 and thecathode 304. In some embodiments, a plurality of anodes 316 and cathode304 may be arranged into a stacked configuration with a separator 306separating each anode 316 and cathode 304. Each negative currentcollector 308 may have one anode 316 attaches to each side; eachpositive current collector 310 may have one cathode 304 attached to eachside. The stacks are immersed in an electrolyte 314 and enclosed in apouch 312. The anode 302 and the cathode 304 may comprise one or morerespective electrode films formed thereon. The number of electrodes ofthe battery 300 may be selected to provide desired device performance.

As shown in FIG. 4, the separator 306 may comprise a single continuousor substantially continuous sheet, which can be interleaved betweenadjacent electrodes of the electrode stack. For example, the separator306 may be shaped and/or dimensioned such that it can be positionedbetween adjacent electrodes in the electrode stack to provide desiredseparation between the adjacent electrodes of the battery 300. Theseparator 306 may be configured to facilitate electrical insulationbetween the anode 302 and the cathode 304, while permitting ionictransport between the anode 302 and the cathode 304. In someembodiments, the separator 306 may comprise a porous material, includinga porous polyolefin material.

The lithium ion battery 300 may include an electrolyte 314, for examplean electrolyte having a composition as described herein. The electrolyte314 is in contact with the anode 302, the cathode 304, and the separator306.

As shown in FIG. 4, the anode 302, cathode 304 and separator 306 of thelithium ion battery 300 may be enclosed in a housing comprising a pouch312. In some embodiments, the pouch 312 may comprise a flexiblematerial. For example, the pouch 312 may readily deform upon applicationof pressure on the pouch 312, including pressure exerted upon the pouch312 from within the housing. In some embodiments, the pouch 312 maycomprise aluminum. For example, the pouch 312 may comprise a laminatedaluminum pouch.

In some embodiments, the lithium ion battery 300 may comprise an anodeconnector (not shown) and a cathode connector (not shown) configured toelectrically couple the anodes and the cathodes of the electrode stackto an external circuit, respectively. The anode connector and a cathodeconnector may be affixed to the pouch 312 to facilitate electricalcoupling of the battery 300 to an external circuit. The anode connectorand the cathode connector may be affixed to the pouch 312 along one edgeof the pouch 312. The anode connector and the cathode connector can beelectrically insulated from one another, and from the pouch 312. Forexample, at least a portion of each of the anode connector and thecathode connector can be within an electrically insulating sleeve suchthat the connectors can be electrically insulated from one another andfrom the pouch 312.

EXAMPLES

The below example processes for anode fabrication generally includemixing components together, casting those components onto a removablesubstrate, drying, curing, removing the substrate, then pyrolyzing theresulting samples. N-Methyl-2-pyrrolidone (NMP) was typically used as asolvent to modify the viscosity of any mixture and render it castableusing a doctor blade approach.

Example 1

In Example 1, a polyimide liquid precursor (PI 2611 from HD Microsystemscorp.), graphite particles (SLP30 from Timcal corp.), conductive carbonparticles (Super P from Timcal corp.), and silicon particles (from AlfaAesar corp.) were mixed together for 5 minutes using a Spex 8000Dmachine in the weight ratio of 200:55:5:20. The mixture was then castonto aluminum foil and allowed to dry in a 90° C. oven, to drive awaysolvents, e.g., NMP. This is followed by a curing step at 200° C. in ahot press, under negligible pressure, for at least 12 hours. Thealuminum foil backing was then removed by etching in a 12.5% HClsolution. The remaining film was then rinsed in DI water, dried and thenpyrolyzed around an hour at 1175° C. under argon flow. The processresulted in a composition of 15.8% of PI 2611 derived carbon, 57.9% ofgraphite particles, 5.3% of carbon resulting from Super P, and 21.1% ofsilicon by weight.

The resulting electrodes were then tested in a pouch cell configurationagainst a lithium NMC oxide cathode. A typical cycling graph is shown inFIG. 5.

Example 2

In Example 2, silicon particles (from EVNANO Advanced Chemical MaterialsCo., Ltd.) were initially mixed with NMP using a Turbula mixer for aduration of one hour at a 1:9 weight ratio. Polyimide liquid precursor(PI 2611 from HD Microsystems corp.), graphite particles (SLP30 fromTimcal corp.), and carbon nanofibers (CNF from Pyrograf corp.) were thenadded to the Si:NMP mixture in the weight ratio of 200:55:5:200 andvortexed for around 2 minutes. The mixture was then cast onto aluminumfoil that was covered by a 21 μm thick copper mesh. The samples werethen allowed to dry in a 90° C. oven to drive away solvents, e.g., NMP.This was followed by a curing step at 200° C. in a hot press, undernegligible pressure, for at least 12 hours. The aluminum foil backingwas then removed by etching in a 12.5% HCl solution. The remaining filmwas then rinsed in DI water, dried and then pyrolyzed for around an hourat 1000° C. under argon. The process resulted in a composition of 15.8%of PI 2611 derived carbon, 57.9% of graphite particles, 5.3% of CNF, and21.1% of silicon by weight.

The resulting electrodes were then tested in a pouch cell configurationagainst a lithium NMC oxide cathode. A typical cycling graph is shown inFIG. 6.

Example 3

In Example 3, polyimide liquid precursor (PI 2611 from HD Microsystemscorp.), and 325 mesh silicon particles (from Alfa Aesar corp.) weremixed together using a Turbula mixer for a duration of 1 hour in theweight ratios of 40:1. The mixture was then cast onto aluminum foil andallowed to dry in a 90° C. oven to drive away solvents, e.g., NMP. Thiswas followed by a curing step at 200° C. in a hot press, undernegligible pressure, for at least 12 hours. The aluminum foil backingwas then removed by etching in a 12.5% HCl solution. The remaining filmwas then rinsed in DI water, dried and then pyrolyzed around an hour at1175° C. under argon flow. The process resulted in a composition of 75%of PI 2611 derived carbon and 25% of silicon by weight.

The resulting electrodes were then tested in a pouch cell configurationagainst a lithium NMC Oxide cathode. A typical cycling graph is shown inFIG. 7.

Example 4

In Example 4, silicon microparticles (from Alfa Aesar corp.), polyimideliquid precursor (PI 2611 from HD Microsystems corp.), graphiteparticles (SLP30 from Timcal corp.), milled carbon fibers (from FibreGlast Developments corp.), carbon nanofibers (CNF from Pyrograf corp.),carbon nanotubes (from CNANO Technology Limited), conductive carbonparticles (Super P from Timcal corp.), conductive graphite particles(KS6 from Timca corp.) were mixed in the weight ratio of20:200:30:8:4:2:1:15 using a vortexer for 5 minutes. The mixture wasthen cast onto aluminum foil. The samples were then allowed to dry in a90° C. oven to drive away solvents, e.g., NMP. This was followed by acuring step at 200° C. in a hot press, under negligible pressure, for atleast 12 hours. The aluminum foil backing was then removed by etching ina 12.5% HCl solution. The remaining film was then rinsed in DI water,dried and then pyrolyzed for around an hour at 1175° C. under argon. Theprocess resulted in a composition similar to the original mixture butwith a PI 2611 derived carbon portion that was 7.5% the original weightof the polyimide precursor.

The resulting electrodes were then tested in a pouch cell configurationagainst a lithium NMC oxide cathode. A typical cycling graph is shown inFIG. 8.

Example 5

In Example 5, polyimide liquid precursor (PI 2611 from HD Microsystemscorp.), and silicon microparticles (from Alfa Aesar corp.) were mixedtogether using a Turbula mixer for a duration of 1 hours in the weightratio of 4:1. The mixture was then cast onto aluminum foil covered witha carbon veil (from Fibre Glast Developments Corporation) and allowed todry in a 90° C. oven to drive away solvents, e.g., NMP. This wasfollowed by a curing step at 200° C. in a hot press, under negligiblepressure, for at least 12 hours. The aluminum foil backing was thenremoved by etching in a 12.5% HCl solution. The remaining film was thenrinsed in DI water, dried and then pyrolyzed around an hour at 1175° C.under argon flow. The process resulted in a composition of approximately23% of PI 2611 derived carbon, 76% of silicon by weight, and the weightof the veil being negligible.

The resulting electrodes were then tested in a pouch cell configurationagainst a lithium nickel manganese cobalt oxide (NMC) cathode. A typicalcycling graph is shown in FIG. 9.

Example 6

In Example 6, polyimide liquid precursor (PI 2611 from HD Microsystemscorp.), graphite particles (SLP30 from Timcal corp.), and siliconmicroparticles (from Alfa Aesar corp.) were mixed together for 5 minutesusing a Spex 8000D machine in the weight ratio of 200:10:70. The mixturewas then cast onto aluminum foil and allowed to dry in a 90° C. oven, todrive away solvents (e.g., NMP). The dried mixture was cured at 200° C.in a hot press, under negligible pressure, for at least 12 hours. Thealuminum foil backing was then removed by etching in a 12.5% HClsolution. The remaining film was then rinsed in DI water, dried and thenpyrolyzed at 1175° C. for about one hour under argon flow. The processresulted in a composition of 15.8% of PI 2611 derived carbon, 10.5% ofgraphite particles, 73.7% of silicon by weight.

The resulting electrodes were then tested in a pouch cell configurationagainst a lithium NMC oxide cathode. The anodes where charged to 600mAh/g each cycle and the discharge capacity per cycle was recorded. Atypical cycling graph is shown in FIG. 10.

Example 7

In Example 7, PVDF and silicon particles (from EVNANO Advanced ChemicalMaterials Co), conductive carbon particles (Super P from Timcal corp.),conductive graphite particles (KS6 from Timcal corp.), graphiteparticles (SLP30 from Timcal corp.) and NMP were mixed in the weightratio of 5:20:1:4:70:95. The mixture was then cast on a copper substrateand then placed in a 90° C. oven to drive away solvents, e.g., NMP. Theresulting electrodes were then tested in a pouch cell configurationagainst a lithium NMC Oxide cathode. A typical cycling graph is shown inFIG. 11.

Example 8

Multiple experiments were conducted in order to find the effects ofvarying the percentage of polyimide derive carbon (e.g. 2611c) whiledecreasing the percentage of graphite particles (SLP30 from Timcalcorp.) and keeping the percentage of silicon microparticles (from AlfaAesar corp.) at 20 wt. %.

As shown in FIGS. 12A and 12B, the results show that more graphite andless 2611c was beneficial to cell performance by increasing the specificcapacity while decreasing the irreversible capacity. Minimizing 2611cadversely affected the strength of the resultant anode so a value closeto 20 wt. % can be preferable as a compromise in one embodiment.

Example 9

Similar to example 8, if 2611c is kept at 20 wt. % and Si percentage isincreased at the expense of graphite particles, the first cycledischarge capacity of the resulting electrode is increased. FIG. 13shows that a higher silicon content can make a better performing anode.

Example 10

When 1 mil thick sheets of polyimide are pyrolized and tested inaccordance with the procedure in Example 1. The reversible capacity andirreversible capacity were plotted as a function of the pyrolysistemperature. FIG. 14 indicates that, in one embodiment, it is preferableto pyrolyze polyimide sheets (Upilex by UBE corp) at around 1175° C.

Example 11

The photograph and scanning electron microscope (SEM) micrographs belowshow an example of the composite anode film.

FIG. 15 is a photograph of a 4.3 cm×4.3 cm composite anode film withouta metal foil support layer. The composite anode film has a thickness ofabout 30 microns and has a composition of about 15.8% of PI 2611 derivedcarbon, about 10.5% of graphite particles, and about 73.7% of silicon byweight.

FIGS. 16-21 are SEM micrographs of a composite anode film. Thecompositions of the composite anode film were about 15.8% of PI 2611derived carbon, about 10.5% of graphite particles, and about 73.7% ofsilicon by weight. FIGS. 16 and 17 show before being cycled (theout-of-focus portion is a bottom portion of the anode and the portionthat is in focus is a cleaved edge of the composite film). FIGS. 18, 19,and 20 are SEM micrographs of a composite anode film after being cycled10 cycles, 10 cycles, and 300 cycles, respectively. The SEM micrographsshow that there is not any significant pulverization of the silicon andthat the anodes do not have an excessive layer of solid electrolyteinterface/interphase (SEI) built on top of them after cycling. FIG. 21are SEM micrographs of cross-sections of composite anode films.

Example 12

Described below are measured properties of example silicon particles.These examples are discussed for illustrative purposes and should not beconstrued to limit the scope of the disclosed embodiments.

FIG. 22 is an x-ray powder diffraction (XRD) graph of the sample siliconparticles. The XRD graph suggests that the sample silicon particles weresubstantially crystalline or polycrystalline in nature.

FIGS. 23-26 are SEM micrographs of the sample silicon particles.Although the SEM micrographs appear to show that the silicon particlesmay have an average particle size greater than the measured averageparticle size of about 300 nm, without being bound by theory, theparticles are believed to have conglomerated together to appear to belarger particles.

FIG. 27 is a chemical analysis of the sample silicon particles. Thechemical analysis suggests that the silicon particles were substantiallypure silicon.

FIGS. 28A and 28B are example particle size histograms of twomicron-sized silicon particles with nanometer-sized features. Theparticles were prepared from a FBR process. Example silicon particlescan have a particle size distribution. For example, at least 90% of theparticles may have a particle size, for example, a diameter or a largestdimension, between about 5 μm and about 20 μm (e.g., between about 6 μmand about 19 μm). At least about 50% of the particles may have aparticle size between about 1 μm and about 10 μm (e.g., about 2 μm andabout 9 μm). Furthermore, at least about 10% of the particles may have aparticle size between about 0.5 μm and about 2 μm (e.g., about 0.9 μmand about 1.1 μm).

Example 13

FIG. 29 is a plot of discharge capacity during cell cycling comparingtwo types of example silicon particles. The performance of four samplesof silicon particles (micron-sized particles with nanometer-sizedfeatures) prepared by the FBR process are compared with five samples ofsilicon particles prepared by milling-down larger silicon particles.Thus, certain embodiments of silicon particles with the combinedmicron/nanometer geometry (e.g., prepared by the FBR process) can haveenhanced performance over various other embodiments of silicon particles(e.g., micron-sized silicon particles prepared by milling down fromlarger particles). The type of silicon particles to use can be tailoredfor the intended or desired application and specifications.

Example 14

Preparation of high-FEC electrolytes (electrolytes #1-10 and #12-13):Monofluoroethylene carbonate (FEC), ethyl methyl carbonate (EMC), andoptionally 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether orethylene carbonate (EC) were mixed together at volume ratios given inTable 1. Lithium hexafluorophosphate (LiPF₆) salt was dissolved in eachof the prepared solvents to a concentration of 1.0 M so as to preparehigh FEC electrolytes. Additive(s), such as adiponitrile (AN), lithiumdifluoro(oxalate)borate (LiDFOB), 1,1,2,2-tetrafluoroethyl2,2,3,3-tetrafluoropropyl ether, trimethoxymethylsilane (MTMS), 1,3propanesultone (PS), trimethyl phosphate (TMP), and/or succinonitrile(SN), were added to the electrolyte solution at weight per volumepercentages given in Table 1.

Preparation of electrolytes without FEC (electrolyte #11): Ethylenecarbonate (EC) and ethyl methyl carbonate (EMC) were mixed with eachother at volume ratios of 1:3. Lithium hexafluorophosphate (LiPF₆) saltwas dissolved in the prepared solvent to a concentration of 1.0 M so asto prepare the electrolyte with EC.

TABLE 1 1,1,2,2- tetrafluoroethyl 2,2,3,3- FEC EMC tetrafluoropropyl ECAdditive(s) Electrolyte (vol %) (vol %) ether (vol %) (vol %) (wt/vol %)1 40 60 0 0 — 2 20 60 20 0 — 3 40 60 0 0 2 AN 4 40 60 0 0 1 LiDFOB 5 4060 0 0 5 1,1,2,2- tetrafluoroethyl 2,2,3,3- tetrafluoropropyl 6 40 60 00 1 MTMS 7 40 60 0 0 5 1,1,2,2- tetrafluoroethyl 2,2,3,3-tetrafluoropropyl + 2 PS 8 40 60 0 0 2 PS 9 40 60 0 0 1 TNP 10 40 60 0 01 SN 11 0 75 0 25 — 12 20 60 0 20 — 13 30 60 0 10 —

Fabrication of Conventional Silicon Anode (Si:C65:PAA): Poly(acrylicacid) (PAA) binder was prepared by mixing 6 wt % of PAA (Polysciences,Inc.) with water. Silicon particles and graphite (C65 from Timcal) wereadded to the PAA solution and were mixed together using an overheadmixer with resulting weight ratios of 70:15:15. The mixture was thencast onto a negative current collector (Cu foil) and dried at 90° C. for1 hour, followed by a 120° C. vacuum dry overnight.

Fabrication of Composite Film Anode (PI2611:SLP30:Si:NMP): Polyimideliquid precursor (PI 2611 from HD Microsystems corp.), graphite (SLP30from Timcal), and silicon particles were mixed together using a ballmill mixer in N-methyl pyrrolidone (NMP) solution with the weight ratiosof 53.55:2.68:18.74:25.03. The mixture was then cast onto Mylar film andallowed to dry in a coating machine. This was followed by a curing stepat 200° C. for at least 14 hours. The remaining film was then pyrolyzedfor around an hour at 1175° C. under argon flow to form a monolithicself-supported film. The film was then attached to copper foil using apolyamide imide polymer adhesive using heat lamination at 575° F. for0.5 minute in a hot press.

Fabrication of LiCoO₂ Cathode (PVDF:C65:KS6:LiCoO2): Polyvinylidenefluoride (PVDF) binder was prepared by mixing PVDF (Solef 5130) withNMP. Lithium cobalt oxide (LC420H-B, ShanShan), carbon black (Super C 65from Timcal), and graphite (KS6 from Timcal) were added to the PVDFsolution and were mixed together using an overhead mixer with theresulting weight ratios of 2.16:2.16:0.68:95. The mixture was then castonto a positive current collector (Al foil) and allowed to dry in acoating machine.

Fabrication of Batteries: lithium ion batteries having a structure shownin FIG. 4 were fabricated using the electrolytes prepared as describeabove in this example. Specifically, a negative electrode formed of anegative current collector (Cu foil) coated with a conventional negativeactive material or attached to a monolithic self-supported silicon film,a porous polyolefin separator, and a positive electrode formed of apositive electrode current collector (Al foil) coated with LiCoO₂ as apositive active material were stacked several times. Then, after tabswere welded to the positive and negative current collectors, the stackwas placed in an aluminum laminate pouch. Next, the prepared electrolytewas injected into the resulting battery pouch and the cell was sealedusing a heat sealer, thus fabricating a battery.

Example 15

The performances of lithium ion batteries with different electrolytesare evaluated. FIG. 30 compares the capacity retention performances of alithium ion battery having a composite film anode and a lithium ionbattery having a conventional silicon anode as described above inExample 14, both containing electrolyte #1 (see Table 1). The capacityretention performances of the batteries were evaluated at around roomtemperature (e.g., a temperature of about 20° C. to about 25° C.). Thedischarge characteristic of the batteries was evaluated by the ratio ofthe capacity upon the first discharge at a current of about 0.2 C(C-rate of about 0.2 C), or a discharge rate at which the rated capacityof the batteries is fully discharged in about 5 hours, to capacity uponthe final discharge at 0.2 C. As shown in FIG. 30, the battery that hasthe composite film anode demonstrated significantly improved capacityretention as compared to the battery with the conventional anode.

Example 16

FIG. 31 shows the comparative results of capacity retention performancesof lithium ion batteries having the composite film anode as describedabove in different electrolytes—electrolytes #1, 11, 12 and 13 (seeTable 1). As shown in FIG. 31, the battery containing the non-FECcontaining electrolyte demonstrated significantly lower capacityretention than the batteries containing an FEC-containing electrolyteafter a number of cycles. For example, the battery comprising thenon-FEC containing electrolyte demonstrated a capacity retention of lessthan about 20% of the initial capacitance after about 250 charge anddischarge cycles.

Example 17

Gassing performance of the batteries was also evaluated by measuring thetotal cell thickness increase after subjecting the batteries to about85° C. for a duration of about 4 hours. The thicknesses were measuredalong a mid-point along a length of the respective pouch cell housingprior to exposing to the heat and after 4 hours at about 85° C. Thethickness measurements were performed while the batteries were fullycharged. Prior to thickness measurements before exposing the batteriesto the elevated temperature at about 85° C., the batteries were chargedand discharged twice. Two batteries having the composite film anode withdifferent electrolytes (electrolytes #11 and 12, see Table 1) weretested. FIG. 32A shows the initial cell thicknesses of the two batterycells, and FIG. 32B shows the final thickness after placing the cell in85° C. for 4 hours. The changes in the thickness of the cells aredisplayed in FIG. 32C. As shown in FIG. 32C, the lithium ion batterieswith electrolytes #11 and 12 had comparable gas generation performance.This indicates that the addition of FEC to electrolyte containing EC andEMC does not affect the gassing performance.

The gassing performances of batteries having high-FEC electrolytecontaining no EC (electrolytes #1-10 in Table 1) were also evaluated.The batteries all included a composite film anode. FIG. 33A shows theinitial thicknesses of the lithium ion batteries prior to initiating thegas generation performance evaluation. FIG. 33B shows the thicknesses ofthe lithium ion batteries upon completion of the gas generationevaluation, while FIG. 33C shows the changes in thicknesses for therespective lithium ion batteries. While eliminating EC from theelectrolyte helps to reduce gassing, addition of another co-solvent,such as 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether orvarious additives further reduces the gassing.

Various embodiments have been described above. Although the inventionhas been described with reference to these specific embodiments, thedescriptions are intended to be illustrative and are not intended to belimiting. Various modifications and applications may occur to thoseskilled in the art without departing from the true spirit and scope ofthe invention as defined in the appended claims.

What is claimed is:
 1. An energy storage device, comprising: a firstelectrode; a second electrode; a separator between the first electrodeand the second electrode; and an electrolyte in contact with the firstelectrode, the second electrode, and the separator, wherein theelectrolyte comprises at least two co-solvents, at least one of theco-solvents comprising fluoroethylene carbonate; wherein the electrolyteis free of non-fluorine containing cyclic carbonates; wherein theelectrolyte comprises 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropylether; and wherein at least one of the first electrode and the secondelectrode comprises silicon particles, wherein said silicon particlesare substantially pure silicon.
 2. The device of claim 1, wherein theelectrolyte is free of ethylene carbonate, propylene carbonate,gamma-butyrolactone, and vinylene carbonate.
 3. The device of claim 1,wherein the fluoroethylene carbonate in the electrolyte has aconcentration from about 10% to about 90% by volume.
 4. The device ofclaim 3, wherein the fluoroethylene carbonate in the electrolyte has aconcentration from about 30% to about 90% by volume.
 5. The device ofclaim 3, wherein the fluoroethylene carbonate in the electrolyte has aconcentration from about 20% to about 50% by volume.
 6. The device ofclaim 1, wherein the electrolyte further comprises at least one ofadiponitrile, succinonitrile, 1,3-propane sultone, trimethylphosphate,methyl trimethoxy silane, lithium difluoro(oxalato)borate, and lithiumbis-(oxalato)borate.
 7. The device of claim 1, wherein the1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether in theelectrolyte has a concentration from about 10% to about 90% by volume.8. The device of claim 7, wherein the 1,1,2,2-tetrafluoroethyl2,2,3,3-tetrafluoropropyl ether in the electrolyte has a concentrationfrom about 20% to about 50% by volume.
 9. The device of claim 1, whereinthe 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether in theelectrolyte has a concentration of up to 10% by weight.
 10. The deviceof claim 1, wherein at least one of the first electrode and the secondelectrode comprises a composite material, the composite materialcomprising more than about 50% by weight of silicon particles.
 11. Thedevice of claim 1, wherein at least one of the first electrode and thesecond electrode comprises a composite material film, the filmcomprising: greater than 0% and less than about 90% by weight of siliconparticles, and greater than 0% and less than about 90% by weight of oneor more types of carbon phases.
 12. The device of claim 11, wherein atleast one of the one or more types of carbon phases is a continuousphase that holds the composite material film together such that thesilicon particles are distributed throughout the composite materialfilm.